In this specification and claims the term “perchlorate” denotes the perchlorate anion and salts thereof.
In some industrial processes, perchlorates are produced as the main product. For example, ammonium perchlorate manufactured for use in applications involving explosives and rocket fuels. In other chemical processes, such as the electrolytic production of sodium chlorate, perchlorate is generated as a byproduct and makes its way to downstream processes, often included as an impurity in the final product.
Perchlorate is a stable and highly water-soluble entity which exhibits high mobility in underground aquifers. In the past decade, the regulatory and public profile of perchlorate has been increasing, mainly driven by remediation of rocket fuel related sites in the United States and recognition of potential health effects resulting from its exposure through drinking water. As a result, governments are gradually moving towards the implementation of more stringent guidelines and regulations to minimize its impact on the environment.
Significant research efforts from both public and private organizations have resulted in the development of techniques to remove perchlorate. However, many of the methods have been specifically developed to remove low concentration of perchlorate in water only, and no work has focused on the removal of high perchlorate content from complex ionic solution matrices, such as the sodium chlorate electrolyte solution, which contains several components.
Sodium chlorate is the primary feedstock used for the production of chlorine dioxide, used in pulp bleaching, as well as the key material feedstock for sodium chlorite production.
Chlorine dioxide is produced by reacting sodium chlorate with an acid, generally, sulfuric or hydrochloric acid. In some plants, the sodium chlorate is supplied as crystals, and in fewer occasions as an aqueous solution from a merchant plant. Many pulp and paper facilities, however, produce chlorine dioxide by the so-called “integrated process”. In the closed loop “integrated process”, a sodium chlorate solution is produced by an on-site electrolytic system and fed directly to a chlorine dioxide generation unit. This produces chlorine dioxide gas and a partially depleted sodium chlorate liquor, which is rich in sodium chloride and recycled back to the electrolytic system for enrichment.
The electrochemical production of sodium chlorate by the electrolysis of sodium chloride solution in an electrolysis cell can generally be described by the following overall reaction:NaCl+3H2O→NaClO3+3H2 
In more detail, the electrolysis of sodium chloride produces chlorine at the anode and hydroxide plus hydrogen at the cathode; and, in an undivided electrolytic cell, the produced chlorine and hydroxide react to form hypochlorous acid and hypochlorite ion, which further react in solution to form chlorate. To avoid excessive production of oxygen through anodic side reactions, the sodium chloride concentration is controlled resulting in a solution containing mainly sodium chlorate and sodium chloride compounds. This mixture is typically referred to as “cell liquor”. To further promote efficiency in the electrolytic cells, the addition of a chemical agent such as sodium dichromate is well known in the industry. However, this additive contributes to heavy metal contamination issues due to the chromium content.
In a Sodium Chlorate merchant plant, sodium chlorate crystals are obtained from the cell liquor by selective crystallization. The remaining liquor after selective crystallization of sodium chlorate, typically referred to as “mother liquor” is recirculated to the electrolyzer, along with the addition of sodium chloride, in the form of brine solution, to maintain the sodium chloride level in the solution for optimum electrolytic performance.
There are several undesirable reactions occurring during electrolysis that can lower the overall cell efficiency, thus increasing the net energy consumption. The formation of sodium perchlorate (NaClO4) in the cell, well recognized to be a significant side reaction, occurs by direct electrochemical oxidation of sodium chlorate. The subsequent accumulation of highly water-soluble sodium perchlorate in the cell liquor ultimately reduces the solubility of sodium chloride, which, in turn, directly affects the cell operation and, in particular, the anode performance with respect to by-product oxygen production, “wear” rate or life of expensive electrocatalytic anode coating, and energy consumption through anodic over-potential. Eventually, the perchlorate concentration builds up to a level where it is purged with the final product as crystals or liquid solution, subsequently finding its way to the downstream chlorine dioxide facility and possibly further making its way into the process effluent.
The formation of sodium perchlorate becomes a larger problem when sodium chlorate is produced in an integrated or closed loop chlorine dioxide system, where there is neither a crystal nor liquid purge for the perchlorate. In the integrated or closed loop chlorine dioxide process, significant amounts of sodium perchlorate can accumulate and thereby promote precipitation of salt in the chlorine dioxide generator, which ultimately can lead to unstable generator operation.
In a modern facility, the formation of perchlorate in the electrolyzer is influenced by several factors; including the sodium chloride concentration in the cell liquor, the anode condition and type of electro-catalytic anode coating, the operating current density, and concentration levels of other impurities such as iron and silica. In various facilities, the formation of perchlorate has been observed to vary between 50 and 500 milligrams sodium perchlorate per kilogram sodium chlorate. For a 100 tonne per day sodium chlorate production plant, this is equivalent to 5 to 50 kg/day of sodium perchlorate, which either accumulates within the system and resulting in operational problems, or directly or indirectly finds its way into the environment.
Further to the aforementioned problems associated with accumulation of perchlorate in sodium chlorate process and particularly in a merchant sodium chlorate crystal plant, increased levels of sodium perchlorate will lead to reduction of sodium chloride solubility in the electrolyte, further complicating the flash crystallization operation. For an integrated chlorine dioxide process, as documented in U.S. Pat. No. 5,324,497 (1994), U.S. Pat. No. 3,929,974 (1975), and U.S. Pat. No. 4,086,329 (1978) assigned to Erco, and U.S. Pat. No. 5,458,858 (1995), assigned to Vulcan, a decreased solubility of sodium chloride due to higher levels of sodium perchlorate will cause more sodium chloride to precipitate in the chlorine dioxide generator, resulting in increased down time and higher maintenance costs. Methods for the removal of sodium perchlorate from sodium chlorate liquors are known in the art. The addition of potassium chloride can be used to preferentially precipitate sodium perchlorate as potassium perchlorate from sodium chlorate electrolyte solution, but achieving perchlorate levels below 40 grams per liter (gpl) without significant simultaneous losses of chloride and chlorate remained unresolved.
U.S. Pat. No. 5,063,041, assigned to Eka Nobel AB describes a process to further reduce the sodium perchlorate content in cell liquor by first concentrating a portion of the chlorate liquor by evaporation, followed by cooling and precipitating with the addition of potassium chloride; the solid phase is then separated and the liquid phase is recirculated back to the chlorate process. A 50% reduction in the electrolyte volume could gradually reduce the perchlorate levels down to near 20 gpl, while a 75% or more reduction in the electrolyte volume would be required to achieve 10 gpl sodium perchlorate levels using this approach.
U.S. Pat. No. 5,681,446 assigned to Sterling Pulp Chemicals describes a process using simultaneous addition of calcium chloride and the potassium chloride to a portion of the sodium chlorate mother liquor after the sodium chlorate crystallizer to reduce both sulphate and perchlorate impurities. By treating the mother liquor instead of the electrolyzer cell liquor, costly evaporation is avoided. However, neither experimental results nor operating data is given on the achievable equilibrium levels of sodium perchlorate in the process. Unfortunately, the process produces a mixture of calcium sulphate and potassium perchlorate sludge, significantly contaminated with chemical compounds containing chloride, chlorate and dichromate.
The above processes produce a toxic sludge contaminated with chlorate and dichromate (hexavalent chromium) which would require further treatment before disposal. The treatment process can be complex and disposal costs can be prohibitive.
U.S. Pat. No. 7,250,144 assigned to Tronox LLC describes a process where a secondary crystallization step without the use of chemical treatment is utilized to enhance the removal of perchlorate from sodium chlorate liquors. This multi-stage crystallization process claims to lower the sodium perchlorate content to below 1 gpl. It also claims that a majority of the first crystallizer mother liquor is processed within a secondary vacuum crystallizer where the perchlorate concentration is gradually increased to a high level, while the perchlorate content in the main process is systematically decreased to less than 1 gpl. But operation of a second crystallizer is often associated with substantial capital and operation costs. Also, no detailed description of the quality of the concentrated perchlorate liquor is provided nor is a disposal method mentioned. Hence, since it's a byproduct of a sodium chlorate production system, one skilled in the art can deduce that this perchlorate liquor will contain some quantity of chlorate, chloride and dichromate. Therefore, in addition to the extra crystallization equipment and processing steps the costs involved for the treatment and disposal of a contaminated byproduct would also render the approach to be less economically attractive.
Chemical treatment using potassium chloride and other potassium salts represents a significant operating expense, while the addition of a secondary crystallizer will also add significant capital costs with associated increase of energy requirements. Accordingly, there is still a need in the industry for improved methods for removal of perchlorate from an electrolytic process for producing sodium chlorate.
The use of weak-base anion exchange resins for the removal of anions from ground water is well-known in the field. These weak-base anion resins have a high capacity for monovalent anions such as chloride, hydroxide and bicarbonate; therefore, they're not efficient for the removal of perchlorate ions in a highly ionic matrix and have not been applied in process systems for the production of sodium chlorate. However, strong-base anion exchange resins like Purolite 530E, a specially functionalized resin originally developed by Oak Ridge National Laboratories in the United States, have shown greater selectivity and higher capacity for perchlorate than weak-base anion resins, but they're also much more difficult to regenerate, which often require unconventional recovery methods like those described by U.S. Pat. No. 6,448,299 assigned to U.T. Battelle, LLC “Regeneration of Strong-Base Anion-Exchange Resins by Sequential Chemical Displacement” which describes the regeneration process using a mixture of strongly acidic tetrachloroferrate (FeCl4−) and hydrochloric acid. This technique, although technically viable, is often not economical and requires complex regeneration steps and the use of hazardous chemicals. Furthermore, any disposal of the hazardous regeneration fluids and their inherent chemical incompatibility within the sodium chlorate process that might occur through unintentional contamination, will further limit its practical application.
Amphoteric resins, also known as ion-retardation resins contain both anionic and cationic adsorption sites which are so closely associated that they partially neutralize other's electrical charges as described in detail by U.S. Pat. No. 3,078,140 assigned to The DOW Chemical Company. However, the sites still have sufficient attraction for mobile anions and cations so that the resins will adsorb both cations and anions from solution with which it comes in contact, but the adsorbed ions can be displaced from the resins by the use of water as an eluant. These resins are commercially available from Dow Chemical Company (Trade name: Dowex Retardion 11A8) and Mitsubishi Chemical Corporation (Trade name: Diaion AMP01, Diaion DSR01) and others.
The Dowex Retardion 11A8 resin, also known as snake-in-a-cage type resin contains both weak acid cation and strong base anion functionality within the same resin; ions are separated from each other based on their affinity to the adsorption sites. The Mitsubishi Diaion AMP01 is classified as a betaine type resin, a neutral chemical compound with a positively charged cationic functional group and with a negatively charged functional group. The two resins exhibit similar ion-retarding action.
A paper by Takeshi Matsushita “Sulfate Removal from Brine by Using Amphoteric Ion Exchange Resin” published in the Journal of Ion Exchange, Vol. 7 No. 3 (1996) describes the use of amphoteric resin “Diaion DSR01” containing both quaternary ammonium group (strong base anion) with a carboxyl group (weakly-acidic cation) on a single aromatic group for the separation of sulphate and chlorate from a brine solution. Other applications of amphoteric resins are disclosed by U.S. Pat. No. 6,482,305 assigned to Archer-Daniels-Midland Company (Nov. 19, 2002). This patent illustrates how a combination of several chromatographic separation steps using amphoteric resins can be applied to the chlor-alkali process. One application is for reduction of chloride salt from a moderately strong alkali solution and another application disclosed is for the reduction of chlorate from acidified brine solution.
The use of amphoteric resins for the separation of sodium chloride, sodium chlorate and sodium sulphate in ionic solution has been successfully demonstrated and commercially applied in the chloralkali industry. Depending on the degree of affinity of the various ions to the ion-retardation resins, elution of the adsorbed ions can be achieved by passing demineralised water to fractionate mixtures of highly ionized substances to enable recovery and reuse of the major chemical components. This simpler water “regeneration” is unlike that required for common ion-exchange resins, where the cations or anions are ionically exchanged and held strongly or captured at the exchange sites thus needing the use of regeneration chemicals that can displace the captured ions; and furthermore, the resulting regeneration effluent solutions in conventional “capture” ion exchange systems must also be treated before disposal. Since ion retardation requires only water for “regeneration”, it can be more profitably employed where ion exchange is not economically practical, especially in complex ionic solution matrices, such as the sodium chlorate electrolyte solution.